Epithelial tissues cover or line all body parts both externally and internally. With cells tightly packed together and with little intercellular space, the epithelial tissues form effective barriers between underlying tissues and external environment. Thus, the epithelial tissues are extremely important in regulating the exchange of materials between different body parts. Intercellular junctions are important for the development and functioning of epithelial tissues. In epithelia, tight junctions (or zonula occludens) are the most apical component of the intercellular junction complex (M. G. Farquhar et al., 1963), which also includes adherens junctions, desmosomes and gap junctions (Haraj N. S. et al., 2004).
The classical functions of the tight junctions (TJs) are the regulation of paracellular permeability and the restriction of apical-basolateral intramembrane diffusion of lipids (Matter K et al., 2003). Composed of occludin and various members of the claudin family of proteins (L. Gonzalez-Mariscal et al., 2003), the TJ strands can reject or retard certain solutes on the basis of their size and others on the basis of charge (G. T. Knipp et al., 1997). Thus, the formation of these intercellular junctional complexes between the epithelial cells provides protective barriers to the body against potentially harmful environmental conditions. These barriers, however, also obstruct the delivery of drugs to target tissues.